Shear-type cutoff apparatus for elongated stock such as tubing typically comprises an arrangement of platens which can be driven relative to one another to cause one or more blades to pass through the stock to perform the shearing or severing function. In the case of tubing and certain other shapes, it is highly desirable, if not necessary, to clamp the cutoff apparatus to the workpiece immediately before, during, and immediately after the shearing function to prevent any relative motion between the stock and the cutoff apparatus which might effect the quality of the cut or even tend to break the cutoff blade. A cutoff apparatus of this general type is disclosed in the U.S. Pat. No. 4,108,029 issued Aug. 22, 1978 to Alexander Borzym.
To operate the clamping system of the cutoff apparatus described above, clamping jaws are disposed on respective slide members each of which is provided with a cam follower roller. A depending cam extends between the rollers and varies in effective thickness so as to program relative displacement between the rollers and the slide members to which the rollers are connected. In this fashion longitudinal motion of the cam, usually coordinated with motion of the platen which carries the shearing blade, programs or controls the movement of the slides and the associated clamping jaws.
As illustrated in the aforementioned U.S. Pat. No. 4,108,029, the clamping system is essentially all-mechanical and requires a close tolerance and proper fitting relationship between the various parts thereof as well as between the clamping jaws and the workpiece for proper operation. If a mechanical obstruction prevents the clamping jaws from closing on the workpiece when the cam enters the space between the follower rollers, an overload condition can occur and this is likely, if great enough, to result in a structural failure of one of the parts of the die clamping arrangement.
One approach to the avoidance of the overload circumstances described above is disclosed in U.S. Pat. No. 4,437,374 issued Mar. 20, 1984 to John J. Borzym. In the apparatus described in that patent one of the clamping apparatus follower rollers is mounted for spring-biased travel relative to the die jaw slide member with which it is associated such that the roller is permitted to "give" in a yielding and resilient fashion in the event a mechanical obstruction prevents the die jaws from closing completely.
Although the apparatus disclosed in the aforesaid U.S. Pat. No. 4,437,374 has proven satisfactory, it has been found desirable to use relatively heavy springs to bias the resiliently mounted follower roller so that low level forces produced relatively little displacement between the roller and its associated slide. All springs, as is well known, follow Hooke's Law and produce a proportional displacement in response to even a small load. Moreover springs are responsive to abrupt shock loads of higher magnitude to displace farther than desired and then rebound and this has been found to give rise to unnecessary travel and wear in the clamping apparatus. It would be desirable to utilize an overload system having the characteristics of a solid mechanical connection for normal loads i.e., loads of less than a predetermined level, and having the characteristics of a spring or resilient connection for loads in excess of said predetermined level.